U.S. patent number 5,540,086 [Application Number 08/298,464] was granted by the patent office on 1996-07-30 for oil deterioration sensor.
This patent grant is currently assigned to Kavlico Corporation. Invention is credited to Marcos A. Nassar, Kyong M. Park.
United States Patent |
5,540,086 |
Park , et al. |
July 30, 1996 |
Oil deterioration sensor
Abstract
An oil deterioration sensor includes a housing, a substrate with
a first capacitive plate mounted within the housing, a second
capacitive plate mounted to the housing close to the first
capacitive plate, and a total reference capacitor which includes an
external fixed reference capacitor. The second capacitive plate is
mounted to the housing such that oil freely circulates within a gap
between the first and second capacitive plates thereby defining an
oil deterioration sensor capacitor. The respective capacitances of
the oil deterioration capacitor and the total reference capacitor
provide an engine oil deterioration indication for the oil within
the gap. The first capacitive plate is divided into a major sensing
area, and a minor sensing area covering a relatively small portion
of the total area of the first capacitive plate. The major sensing
area and the second capacitive plate form the oil deterioration
sensor capacitor. The total reference capacitor further includes an
oil deterioration reference capacitor defined by the minor
reference area and the second capacitive plate. The oil
deterioration sensor further includes a temperature sensitive
resistive element thermally connected to the substrate for
providing a temperature adjustment to the engine oil deterioration
indication, and circuitry utilizing the respective capacitances of
the oil deterioration sensor capacitor and the total reference
capacitor to generate the engine oil deterioration indication.
Inventors: |
Park; Kyong M. (Thousand Oaks,
CA), Nassar; Marcos A. (Los Angeles, CA) |
Assignee: |
Kavlico Corporation (Moorpark,
CA)
|
Family
ID: |
23150638 |
Appl.
No.: |
08/298,464 |
Filed: |
August 30, 1994 |
Current U.S.
Class: |
73/53.05;
324/663; 324/71.1; 340/603; 73/54.01 |
Current CPC
Class: |
G01N
27/221 (20130101); G01N 33/2888 (20130101); H01R
13/5045 (20130101) |
Current International
Class: |
G01N
27/22 (20060101); G01N 33/26 (20060101); G01N
33/28 (20060101); G01N 011/00 (); G01N
033/30 () |
Field of
Search: |
;73/53.05,54.01,53.01,61.42
;324/663,672,685,686,689,690,664,670,71.1 ;340/603,631 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Dean R. Harrison and John Dimeff, "Transducers", Rev. Sci.
Instrum., vol. 44, No. 10, Oct. 1973, pp. 1468-1472..
|
Primary Examiner: Brock; Michael J.
Attorney, Agent or Firm: Poms, Smith, Lande & Rose,
P.C.
Claims
What is claimed is:
1. A capacitive oil deterioration sensor comprising:
a threaded metal housing having an inner end for threading into an
oil pan of an automobile or other engine, said inner end including
supports attached thereto;
a heat-conducting substrate mounted within said inner end and
insulated from said housing, said substrate including an inner
surface and an outer surface and a first capacitive plate on said
outer surface;
a second capacitive plate mounted to said housing by said supports
close to but apart from said first capacitive plate such that oil
freely circulates within a gap between said first and second
capacitive plates, said first and second capacitive plates defining
an oil deterioration sensor capacitor;
a total reference capacitor including an external fixed reference
capacitor, said oil deterioration sensor capacitor and said total
reference capacitor providing an indication of viscosity of said
oil within said gap; and
a temperature sensitive resistive element thermally connected to
said inner surface of said substrate for providing a temperature
adjustment to said indication of viscosity in response to a
temperature of said oil.
2. The capacitive oil deterioration sensor of claim 1 wherein said
first capacitive plate is divided into a major sensing area, and a
minor reference area covering a relatively small fraction of a
total area of said first capacitive plate, said major sensing area
and said second capacitive plate forming said oil deterioration
sensor capacitor, said total reference capacitor further including
an oil deterioration reference capacitor defined by said minor
reference area and said second capacitive plate.
3. The capacitive oil deterioration sensor of claim 1 further
including circuitry for generating said indication of viscosity
utilizing capacitances of said oil deterioration sensor capacitor
and said total reference capacitor.
4. The capacitive oil deterioration sensor of claim 3 wherein said
circuitry includes a hybrid circuit located within said
housing.
5. The capacitive oil deterioration sensor of claim 1 further
including a temperature sensitive resistive element thermally
connected to said substrate for providing a temperature adjustment
to said indication of viscosity in response to a temperature of
said oil.
6. The capacitive oil deterioration sensor of claim 1 wherein said
substrate comprises a ceramic material.
7. The capacitive oil deterioration sensor of claim 1 wherein said
first capacitive plate includes gold.
8. The capacitive oil deterioration sensor of claim 1 further
comprising a glass passivation layer over said first capacitive
plate.
9. The capacitive oil deterioration sensor of claim 1 wherein a
distance between said first and second capacitive plates is between
0.010 and 0.020 inches.
10. The capacitive oil deterioration sensor of claim 1 wherein said
first capacitive plate is substantially parallel to said second
capacitive plate.
11. A capacitive oil deterioration sensor comprising:
a threaded metal housing having an inner end for protruding into an
oil pan of an automobile or other engine;
an exposed insulating disk having a first metal capacitive plate on
an outer surface thereof insulated from said housing and mounted
within said inner end of said housing;
a second metal capacitive plate spaced close to but apart from said
first metal capacitive plate for cooperation with said first metal
capacitive plate;
supports for mounting said second metal capacitive plate to permit
free circulation of motor oil between said first and second metal
capacitive plates; and
a temperature sensitive resistance mounted on an inner surface of
said insulating disk on another side of said insulating disk from
said first metal capacitive plate;
whereby changes in a dielectric constant of oil circulating between
said plates may be detected and warning given following
deterioration of said oil.
12. The capacitive oil deterioration sensor of claim 11 wherein
said insulating disk comprises a ceramic material which rapidly
transfers heat to said temperature sensitive resistance.
13. The capacitive oil deterioration sensor of claim 11 wherein
said first metal capacitive plate is divided into a major sensing
area, and a minor reference area covering a relatively small
fraction of a total area of said first metal capacitive plate, an
external reference capacitor is provided, and circuitry is employed
to combine a capacitance of said external reference capacitor with
a capacitance of said minor reference area to provide a total
reference capacitance and to compare said total reference
capacitance to a capacitance of said major sensing area.
14. A capacitive oil deterioration sensor comprising:
a housing having an inner end for protruding into an oil pan of an
automobile or other engine;
a substrate mounted within said inner end and insulated from said
housing, said substrate including a first capacitive plate on an
outer surface of said substrate;
a second capacitive plate mounted to said housing close to but
apart from said first capacitive plate such that oil freely
circulates within a gap between said first and second capacitive
plated, said first and second capacitive plates defining an oil
deterioration sensor capacitor; and
a total reference capacitor including an external fixed reference
capacitor, said oil deterioration sensor capacitor and said total
reference capacitor providing an indication of viscosity of said
oil within said gap.
15. The capacitive oil deterioration sensor of claim 14 wherein
said first capacitive plate is divided into a major sensing area,
and a minor reference area covering a relatively small fraction of
a total area of said first capacitive plate, said major sensing
area and said second capacitive plate forming said oil
deterioration sensor capacitor, said total reference capacitor
further including an oil deterioration reference capacitor defined
by said minor reference area and said second capacitive plate.
16. The capacitive oil deterioration sensor of claim 14 further
including circuitry for generating said indication of viscosity
utilizing capacitances of said oil deterioration sensor capacitor
and said total reference capacitor.
17. The capacitive oil deterioration sensor of claim 14 further
including a temperature sensitive resistive element thermally
connected to said substrate for providing a temperature adjustment
to said indication of viscosity in response to a temperature of
said oil.
18. The capacitive oil deterioration sensor of claim 14 wherein
said substrate comprises a ceramic material.
19. The capacitive oil deterioration sensor of claim 14 wherein
said first capacitive plate includes gold.
20. The capacitive oil deterioration sensor of claim 14 further
comprising a glass passivation layer over said first capacitive
plate.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an oil deterioration sensor and,
more particularly, pertains to an oil deterioration sensor
employing a capacitive transducer with an oil deterioration sensor
capacitor and at least one reference capacitor.
2. Description of the Related Art
As engine oil "breaks down" it is less effective in protecting an
engine from damage caused by friction between engine parts. The
deterioration of engine oil is marked by a decrease in the
viscosity of the oil. The dielectric constant of engine oil
provides an indication of the oil's deterioration or lack thereof.
However, the dielectric constant of oil is also influenced by the
temperature of the oil and by the specific formulation of a given
brand of oil.
Accordingly, an object of the present invention is to provide an
oil deterioration sensor employing a capacitive transducer wherein
the engine oil is utilized as a dielectric medium.
Another object is to provide an oil deterioration sensor which
utilizes a combination of fixed and variable capacitors to provide
an indication of engine oil breakdown for a wide variety of engine
oil formulations.
A further object is to provide an oil deterioration sensor wherein
the variable capacitances are configured such that stray
capacitances are reduced.
Still another object is to provide an oil deterioration sensor
which adjusts the engine oil deterioration measurements to
compensate for the effects of engine oil temperature changes.
SUMMARY OF THE INVENTION
In accordance with a specific illustrative embodiment of the
present invention, an oil deterioration sensor includes a housing,
a substrate with a first capacitive plate mounted within the
housing, a second capacitive plate mounted to the housing close to
the first capacitive plate, and a total reference capacitor which
includes an external fixed reference capacitor. The second
capacitive plate is mounted to the housing such that oil freely
circulates within a gap between the first and second capacitive
plates thereby defining an oil deterioration sensor capacitor. The
respective capacitances of the oil deterioration capacitor and the
total reference capacitor provide an engine oil deterioration
indication for the oil within the gap.
In a further aspect of the present invention, the first capacitive
plate is divided into a major sensing area, and a minor sensing
area covering a relatively small portion of the total area of the
first capacitive plate. The major sensing area and the second
capacitive plate form the oil deterioration sensor capacitor. The
total reference capacitor further includes an oil deterioration
reference capacitor defined by the minor reference area and the
second capacitive plate.
In still another aspect of the present invention, the oil
deterioration sensor further includes a temperature sensitive
resistive element thermally connected to the substrate for
providing a temperature adjustment to the engine oil deterioration
indication, and circuitry utilizing the respective capacitances of
the oil deterioration sensor capacitor and the total reference
capacitor to generate the engine oil deterioration indication.
In accordance with another aspect of the invention, the oil
deterioration sensor may include a threaded metal housing with an
inner end for extending into the oil pan of an engine, with the two
capacitive plates being mounted at the inner end and being open and
spaced apart for the free flow of oil between the capacitive
plates.
DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the invention will become
readily apparent upon reference to the following detailed
description when considered in conjunction with the accompanying
drawings, in which like reference numerals designate like parts
throughout the figures thereof, and wherein:
FIG. 1 is a cross-sectional side view of an oil deterioration
sensor of the present invention;
FIG. 2 is an end view of the oil deterioration sensor along the
2--2 line of FIG. 1;
FIG. 3 is an end view of the oil deterioration sensor along the
3--3 line of FIG. 1;
FIG. 4 is a rear view of the oil deterioration sensor of FIG.
1;
FIG. 5 is an electrical schematic of circuitry within the oil
deterioration sensor for generating an indication of engine oil
deterioration;
FIG. 6 is a plot of the sensor's output indication voltage (VDC) at
room temperature over time (hours); and
FIG. 7 is a plot of the sensor's output indication voltage (VDC)
for various engine oil formulations over temperature
(.degree.C).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a cross-sectional side view of a capacitive oil
deterioration sensor 10 fitted within a wall 12 of an engine oil
reservoir, such as the oil pan of a motorized vehicle. The sensor
10 includes a metal housing 14 with an inner end 16 which extends
into the oil pan and is exposed to the oil therein. The inner end
16 preferably includes a threaded surface portion 18 which is
fitted within a corresponding threaded bore 20 of the oil pan wall
12. A circular gasket 22 made of a sealant such as rubber may also
be positioned between the housing 14 and the wall 12 as shown in
FIG. 1. The wall 12 may be a thick local area in a thinner metal
oil pan to provide additional strength for receiving the oil
deterioration sensor.
The housing 14, and particularly the inner end 16, are precisely
machined to receive a substrate 24. The preferred inner end 16 is
sized to form a cylindrical chamber along the length of the sensor
10. Accordingly, the preferred substrate 24 is cylindrical in shape
with an outer diameter (e.g., 0.6 in.) slightly less than the inner
diameter of the inner end 16. The substrate 24 is preferably
manufactured from an insulating material which efficiently conducts
heat. For example, the substrate 24 may be formed from a ceramic
material such as alumina.
The substrate 24 includes an outer surface 26 and an inner surface
28 on opposing sides thereof, with the substrate 24 being oriented
within the inner end 16 such that the outer surface 26 faces the
interior of the oil pan. A first capacitive plate 30 is formed on
the outer surface 26 and is exposed to the engine oil. The first
capacitive plate 30 is formed from a conductive material. In a
preferred embodiment, the plate 30 is comprised of gold and may
have a thickness of one or two thousandths of an inch. It may be
silk screened on to the ceramic disk 24 in paste form and
fired.
The inner end 16 of the housing 14 additionally includes a
plurality of supports 32 which are sized to secure a second
capacitive plate 34 within the housing 14. The second capacitive
plate 34 is also made from a conductive material (e.g., steel), and
is held stationary by the supports 32 such that the first 30 and
second 34 capacitive plates are substantially parallel. The
distance between the first 30 and second 34 plates is preferably
between 0.010 and 0.020 inches to permit the free circulation of
oil from the oil pan through the space between the plates.
The oil deterioration sensor 10 additionally includes means for
preventing the substrate 24 from sliding within the inner end 16
away from the second capacitive plate 34. For example, the
preventing means may comprise an outer spacer 36 fitted within the
housing 14. Also, a ring seal 38 may be provided to prevent the
engine oil from seeping past the substrate 24 and into the housing
14. The ring seal 38 is comprised of a sealant such as silicone
rubber.
The four supports 32, in addition to securing the second capacitive
plate 34 to the housing 14, are spaced apart around the inner end
16 such that engine oil freely circulates within a gap 40 between
the first capacitive plate 30 and the second icapacitive plate 34.
Accordingly, an oil deterioration sensor capacitor is defined by
the first capacitive plate 30, the second capacitive plate 34 and
the engine oil in the gap 36. The capacitance of the oil
deterioration sensor capacitor varies depending upon the dielectric
constant of the oil between the plates 30 and 34.
Before the operational aspects of the capacitive oil deterioration
sensor 10 are discussed in detail, it should be noted that the
sensor 10 additionally includes a connector shell portion 42 which
is mechanically connected to the housing 14 as shown in FIG. 1. A
plurality of connectors or conductive terminals are secured within
the shell portion 42. A connector support structure 44 and a ring
seal 46 are secured between a conductive retaining ring 48 and the
connector shell portion 42. The retaining ring 48, in turn, is
secured between the support structure 44 and the ring seal 46 on
one side and the outer spacer 36 on the other side. The
aforedescribed mechanical assembly prevents the structure 24, and
thus the first capacitive plate 30, from sliding within the housing
14 relative to the second capacitive plate 34.
The oil deterioration sensor 10 further includes circuitry within
the housing 14 for generating an engine oil deterioration
indication signal. Preferably, some of the elements of the
circuitry are provided in an integrated circuit 50 (e.g., a hybrid
integrated circuit) which is thermally isolated from the substrate
24. The circuitry also includes the oil deterioration sensor
capacitor and a total reference capacitor (discussed below). In a
preferred embodiment, the circuitry also includes other circuit
elements which are necessarily in thermal contact with the
substrate 24. For example, the circuitry also includes a
temperature sensitive resistor 52 which is mechanically attached or
bonded to the inner surface 28 and thermally connected to the
substrate 24.
FIG. 2 is an end view of the oil deterioration sensor 10 along the
2--2 line of FIG. 1. The first capacitive plate 30 is divided into
a major sensing area 54 and a minor reference area 56 covering a
relatively small fraction of a total area of the first capacitive
plate 30. The major sensing area 54 and the second capacitive plate
34 form the oil deterioration sensor capacitor (C.sub..epsilon.)
with the engine oil serving as the dielectric material
therebetween. The sensor may also include a passivation layer 57
formed over the first capacitive plate 30. For example, the
passivation layer 57 may comprise a polyamide coating between
0.0005 and 0.002 inch in thickness.
The total reference capacitor (C.sub.R) mentioned supra includes an
external fixed reference capacitor (C.sub.RF) 58 (FIG. 1) which is
electrically connected to the integrated circuit 50. The total
reference capacitor further includes an oil deterioration reference
capacitor (C.sub.R.epsilon.) which is electrically connected in
parallel with C.sub.RF. The oil deterioration reference capacitor
(C.sub.R.epsilon.) is defined by the minor reference area 56 and
the second capacitive plate 34 with the engine oil also serving as
the dielectric material therebetween. The plate 34 and the housing
14 may be formed of steel, and may be at ground potential.
Accordingly, the following formula approximates the relationship
between the above reference capacitors:
The preferred major sensing area 54 is 0.188 in..sup.2 and may be
generally circular in shape. The preferred minor reference area 56
is positioned within and electrically isolated from the major
sensing area 54, 0.015 in..sup.2 and generally circular in shape.
As shown in FIG. 1, the major sensing area 54 and the minor
reference area 56 are electrically connected to the integrated
circuit 50 by leads 60, 62, respectively.
During assembly of the oil deterioration sensor 10, leads 60, 62
are respectively routed through conduits 64, 66 which are formed in
the substrate 24. The conduits 64,66 are sized only slightly larger
than the leads 60, 62 so that a sealant need not be injected or
otherwise positioned between the leads 60,62 and inner walls of the
conduits 64, 66. FIG. 2 further illustrates that the lead 60 is
electrically connected to the major sensing area 54 by a solder
joint 68. Similarly, the lead 62 is electrically connected to the
minor reference area 56 by a solder joint 70. Solder joints 68, 70
may include silver epoxy (e.g., Heraeus 60-193) and additionally
act to prevent engine oil from seeping into the sensor 10 through
the conduits 64, 66.
FIG. 1 illustrates that the second capacitive plate 34 is
electrically connected to the integrated circuit 50 through the
electrically conductive housing 14, retaining ring 48 and connector
support structure 44. Also, the temperature sensitive resistor 52
is electrically connected to the integrated circuit 50 by leads 72,
74 which are preferably isolated from the leads 60, 62 by an inner
insulating plastic spacer 76.
FIG. 3 is a limited end view of the oil deterioration sensor 10
along the 3--3 line of FIG. 1. The second capacitive plate 34 is
shown secured by four supports 32 which are evenly spaced around
the inner end 16. As may be readily appreciated, the plurality of
supports 32 may comprise a number other than four supports and are
not necessarily evenly spaced around the inner end 16. In fact, any
configuration of supports 32 which allows the free flow of engine
oil into the gap 40 is contemplated.
FIG. 4 is a rear view of the oil deterioration sensor 10 of FIG. 1.
The terminals supported by the connector shell portion 42 include a
ground voltage terminal 78, a supply voltage terminal 80 and an
output voltage terminal 82. The operating supply voltage (V.sub.CC)
applied across terminals 80, 78 and thereby supplied to the
circuitry is 5,000 .+-.0.250 VDC. The engine oil deterioration
indication signal (V.sub.OUT) generated by the circuitry is
measured across terminals 82, 78 at the connector shell portion
42.
A preferred shell portion 42 includes a hexagonal exterior surface
84 which permits a mechanic, vehicle owner, etc. to thread the oil
deterioration sensor 10 into or out of the oil pan bore 20 as
desired with a conventional wrench. Other mechanical structures
facilitating the mounting and insertion of the oil deterioration
sensor 10 into the oil pan of an engine are also contemplated.
FIG. 5 is an electrical schematic of circuitry 200 within the oil
deterioration sensor 10 for generating the engine oil deterioration
indication signal (V.sub.OUT). For the purpose of simplification,
the operating supply voltage (V.sub.CC) and a ground (GND) are
generally shown as being provided to the hybrid 50. As may be
readily appreciated, the dashed-line boundary of the hybrid 50 may
be adjusted as circuit elements are either added to or relocated
externally from the hybrid 50.
Generally, a capacitive transducer must be excited by an
alternating current (AC) carrier voltage. Demodulating electronics
are also needed to generate a direct current (DC) output.
Accordingly, the circuitry 200 includes a diode-quad circuit 202
which is excited by a AC carrier voltage provided by an oscillator
(OSC) 203. The diode-quad circuit 202 further includes diodes D1,
D2, D3 and D4 arranged to form nodes 204, 206, 208, 210 as shown in
FIG. 5. More specifically, the anode of D1 and the cathode of D4
are electrically connected at the node 204, the anode of D2 and the
cathode of D3 are electrically connected at the node 206, the
cathode of D2 and the anode of D4 are electrically connected at the
node 208, and the cathode of D1 and the anode of D3 are
electrically connected at the node 210.
The engine oil deterioration indication signal (V.sub.OUT) is
generated by the circuitry 200 and is described by the following
formula:
The oil deterioration sensor capacitor (C.sub..epsilon.) is
electrically connected across the node 208 and GND. The total
reference capacitor (C.sub.R), which as discussed above comprises
the oil deterioration reference capacitor (C.sub.R.epsilon.) in
parallel with external fixed reference capacitor (C.sub.RF), is
electrically connected across the node 210 and GND. Since the
capacitors C.sub..epsilon., C.sub.R.epsilon. and C.sub.RF are not
included within the hybrid 50, they are shown outside the
dashed-line outlining the hybrid 50.
The oil deterioration reference capacitor (C.sub.R.epsilon.) is
used to minimize the differences between the respective dielectric
breakdown characteristics of various motor oil formulations. The
capacitance of C.sub.RF is chosen such that C.sub..epsilon. is
approximately equal to C.sub.R when fresh motor oil is introduced
into the gap 40. The capacitance of C.sub..epsilon. is 5-15 pF
without motor oil in the gap 40 and 6-20 pF when the gap 40 is
filled with fresh motor oil.
After the motor oil begins to break down, the respective
capacitances of C.sub..epsilon. and C.sub.R both change, with
C.sub..epsilon. being more sensitive to changes in the dielectric
constant of the motor oil than C.sub.R. As described above, it has
been observed that the engine oil deterioration indication signal
(V.sub.OUT) is proportional to (C.sub..epsilon.
-C.sub.R)/(C.sub..epsilon. +C.sub.R). In summary, the circuitry 200
utilizes the respective capacitances of C.sub..epsilon. and C.sub.R
to generate V.sub.OUT.
As shown in FIG. 5, the preferred circuitry 200 also includes a
temperature compensation element 212 and a level set 214 for the
oscillator 203. These additional elements are conventional with the
temperature compensation element 212 regulating the output of the
oscillator 203 under changing environmental conditions. Further
with regard to temperature compensation, the temperature sensitive
resistor 52 is designated as R6 and adjusts V.sub.OUT to compensate
for changes in V.sub.OUT caused by changes in the temperature of
the engine oil. The preferred temperature sensitive resistor 52
(FIG. 1) is a resistive paste sold under the name Sensohm
manufactured by Ferro Corporation of 27 Castillian Drive, Santa
Barbara, Calif. 93117-3092. Sensohm is characterized as providing
10K.OMEGA./unit with the resistor 52 being formed on the substrate
24 with an appropriate amount of Sensohm to measure 36K.OMEGA..
Following application to the substrate as a paste, the resistor 52
is fired to bond it to the inner surface 28 of the disk 24.
The preferred circuitry 200 further includes an external offset
circuit 216 and an external gain circuit 218 which receive and
adjust the output of the diode-quad circuit 202 as required to
provide V.sub.OUT. The aforedescribed signal conditioning may also
be provided by alternative embodiments of the circuitry 200. For
example, an alternative embodiment of the circuitry may include
diode-quad circuits such as those described in "Transducers" by
Dean R. Harrison and John Dimeff, Rev. Sci. Instrum., Vol. 44, No.
10, October 1973 which is herein incorporated by reference. Other
circuits such as those disclosed in U.S. Pat. Nos. 4,227,419 and
4,398,426, assigned to the assignee of this invention, may also be
employed.
FIG. 6 is a plot of the sensor's output indication voltage (VDC) at
room temperature over time (hours) of operation of an internal
combustion engine. Since capacitance across parallel plates is
generally described by the equation
the respective capacitances of C.sub..epsilon. and C.sub.R.epsilon.
are substantially governed by the above equation wherein .epsilon.
is the dielectric constant of the motor oil within the gap 40.
Since the dielectric constant of motor oil increases as the oil
breaks down, the plot of FIG. 6 demonstrates that V.sub.OUT is
proportional to (C.sub..epsilon. -C.sub.R)/(C.sub..epsilon.
+C.sub.R). As discussed above, the capacitance of C.sub.RF is
chosen such that C.sub..epsilon. is approximately equal to C.sub.R
when fresh motor oil is introduced into the gap 40 so that the
engine oil deterioration indication signal (V.sub.OUT) initially
generated by the circuitry 200 measures close to 0.0 VDC. As the
motor continues to run and the oil therein breaks down, it has been
observed that V.sub.OUT increases over time as shown in FIG. 6.
FIG. 7 is a plot of V.sub.OUT for various engine oil formulations
over engine oil temperature. The data points corresponding to each
particular engine oil formulation are respectively represented by a
different type of indicia. The following table shows which indicia
correspond to each of the tested engine oil formulations.
______________________________________ Brand Indicia
______________________________________ A .box-solid. B .quadrature.
C + D .times. E * F .tangle-solidup.
______________________________________
V.sub.OUT for each of the tested formulations was observed to
increase or decrease over temperature without the adjustment
provided by the temperature sensitive resistor 52. As may be
readily appreciated, the resistor 52 is particularly useful in
accommodating most of the tested engine oil formulations where the
temperature effects were observed to be substantially linear in
nature. Brand B through F were well known nationally distributed
brands of motor oil, which Brand A was a cheap local unknown brand.
It is believed that one of the chemical additions in Brand A caused
the unusual characteristic shown for plot A.
In conclusion, it is to be understood that the foregoing detailed
description and the accompanying drawings illustrate the principles
of the invention. However, various changes and modifications may be
employed without departing from the spirit and scope of the
invention. Thus, by way of example and not of limitation, the
housing 14 could be formed of a high strength engineered plastic
instead of steel, and the second capacitive plate could be in the
form of a coating on the inside of an insulating plate, with
suitable electrical connections to the hybrid circuit being
provided. Also instead of the ceramic disk 24 the inner capacitive
plate could be formed as a separate metallic plate, electrically
isolated from the facing plate and from the housing if the housing
is metallic; and the temperature sensitive resistor could be
mounted on the opposite surface of this plate, electrically
isolated by a thin electrically insulating but thermally conductive
layer. Other mechanical and electrical changes of a comparable
nature could also be made. Accordingly, the present invention is
not limited to the specific form shown in the drawings and
described in detail hereinabove.
* * * * *